Method of controlling superheat temperature



atenteoi at. as, test 2,100,190 METHOD or CONTROLLING strrnnnnn'r" George P. Jackson, Flushing, N. Y., assignor to Combustion Engineering Company, Inc., New

York, N. Y.

Application March 2, 1937, Serial No. 128,571

11 Claims. (Cl. 122-459) The present invention relates to an improved methodof controlling the temperature of superheated steam particularly in boilers provided with superheaters of the convection type.

Present day commercial requirements demand boilers capable of generating large quantities of steam and superheating it to high temperatures. Various arrangements have been pro 'posed to provide suflicient volumes of gases for generating and superheating steam at both high and low loads in a boiler. ,One such arrangements is shown in Bennis Patent 1,580,776, issued April 13-, 1926, wherein burners supplying fuel near the upper portion of a furnace chamber 15 operatein conjunction with a stoker in the lower portion of the furnace. In the boiler of Gordon Patent 1,896,233, issued February '7, 1933, the major portion ofthe fuel is introduced from the top of the furnaceand because the flames are directed downwardly he also provides auxiliary burners to heat wall tubes which otherwise would not receive their share of heat due to the path- ,of travel of the main flame. Yarrow, 1,625,759 of April 19, 1927', shows an arrangement in which 5 auxiliary burners are provided for heating the .35 loads, the rate of fuel supply'being increased with rise in steam demand and being decreased as the steam demand fall's; that is, as the load rises the auxiliary firing means are brought into operation or the rate of firing is increased and 40 vice versa.

In the operation of boilershaving convection type superh'eaters the temperature of the superheated steam tends to rise with increases in steam demand. This condition is brought about as a 45 consequence of the flow of increased quantities of steam through the superheater together with increased volumes of gases at hotter temperatures which yield proportionally less heat to generating surfaces in advance of the superheater than 50 at low boiler loads, thus causing relatively hotter gases in larger volume to contact the superheater.

It is desirable to maintain a substantially constant degree of superheat over a wide range of boiler loads but, where a boiler is designed to 05 provide the desired superheated steam temperate ing a substantially constant superheated steam temperature for a wide range of boiler loads as, for example, where the steam demand at' high loads is doublethat at low loads or greater. 15

Incarrying out the invention the furnace of a boiler is provided with two sets of fuel burners located respectively near its top and bottom.

The gases from the two sets of burners pass over the main generating elements of 'the boiler and 20 the superheating elements, both being located above a furnace of substantial height. The lower set of burners produce gases imparting some Fig. 1 is a diagrammatical elevational view of a boiler provided with firing means adapted to be operated in accordance with the method of the present invention in order to maintain a substantially constant superheated steam temperature over a wide range of boiler loads.

Fig. 2 is a sectional view on the line 2-2 in Fig. 1. I

Referring to Fig. 1, the boiler has a furnace chamber In of substantial height above which are located the main generating elements and the superheating elements. The main generating elements comprise two banks H and 12 of enerating tubes connected between the lower or water drum l3 and the upper steam drums l4, l5. The superheating elements I6 are mounted within or between tube rows of the bank II and partially shielded by one or more. of the outermost rows of tubes of the bank. The walls of the furnace chamber are provided with steam g nerating tubes l1 and similar tubes I'Ia extend along the upper portion of the side walls of the boiler, all these tubes being connected into the boiler circulating system. Near the bottom of the furnace is a set of burners l8 supplied with pulverized coal (for example) by a mill i9. At the upper portion of the furnace is another set of burners 20 which may receive fuel from a separate mill, or from the mill i9 through a conduit 2| controlled by a valve 22 so that part of the fuel from the mill may be diverted from burners l8 to the burners 20.

As indicated in Fig, 2 the, burners l8 are preferably arranged to provide a'vortically ascend- .aginary circle in the furnace.

ing column of burning gases in the furnace by being mounted in opposite walls in offset relation so that streams of fuel and air projected set relation.

In the operation of the boiler all the fuel that is needed to generate and superheat to the desired temperature the amount of steam required at high loads is supplied to the burners l8. Inasmuch as burners l8 are arranged in offset relation so as to project the fuel in directions tangential to a circle within the furnace, a swirling column of ascending gases is produced. These gases sweep the generating tubes H on the furnace walls and give up considerable of their heat by convection to these walls as well as by radiation. Thus, the major part ofthe amount of steam required at high loads is generated in the water wall i1. As the gases pass from the furnace and over the generating tubes l i l2 and the superheater the remainder of the total amount of steam required at high loads is generated and the latter is superheated to the predetermined temperature.

As the demand for steam from the boiler falls, the rate of operation of the main burners ill at the lowerportion of the furnace is decreased.

Consequently the amount of heat absorbed by tubes I! by radiation and the amount of heat imparted by convection to these wall tubes is also diminished by the reduction of the flame envelope so that less steam is generated in the tubes l1.

The products of combustion coming from burners l8 then arrive at the boiler tubes ll, l2 and the superheater at substantially lower temperatures so that a drop in superheat results. However, according to the invention, the firing rate of the burners I8 is reduced a nd burners 20 are placed in operation and proportionately increased to maintain the quantity'of gases produced by both sets of burners at a temperature sufliciently high to generate and superheat the amount of steam required at the lower load. The burners 20 are mounted near the top of the furnace a sufficient distance from the bank i l to assure complete combustion of the fuel in its path from burner to bank but sufiiciently close that the products of combustion therefrom do not impart a substantial amount of heat to the wall tubes E7. The upper burners 20 are so located and directed that they jet their products of combustion into the gases rising from the lower burners i8 and mix therewith. The temperature of the mixture is higher than that of the rising gases from the burners l8 and lower than that of the-products of combustion from the burners 20. p

The change-in the amount of fuel consumed may be effected by reducing the rate of operation of the mill l9 and diverting part of the lesser amount of fuel to'the burners 20 by regulating the valve 22 in the conduit 2! leading from the 'main conduit for burners it to the burners 20.

However, it is obvious that the burners. 20 might be supplied by a separate mill and the respective rates of operation of the two mills controlled in suitable manner to vary the rate of fuel supply as set forth above.-

The burners l8 preferably have ample capa ity to alone provide an adequate quantity of gases at a temperature sufiiciently high to generate and superheat the steam to the desired temperature at maximum loads. However, in some instances it-may be desirable to operate the burners it at less than their capacity and supply the remaining quantity of gases by means of the burners 20.

Thus, as described above, the amount of heat absorbed from the gases by the Wall tubes H is changed by varying the location within the furnaceof a substantial portion of the heat liberated from the burning fuel with respect to the furnace outlet by apportioning the fuel between burners i8 and 20 in order to maintain the superheat temperature substantially constant.

Where the above method of superheat temperature control is applied to a boiler subject to a very high maximum steam demand, the requirements of auxiliary burners 20 to maintain the superheat temperature at lower loads may become so great that the products of combustion therefrom are too quantitative to satisfactorily mix with those rising from the burners it. When this occurs the products from the burners 20 may travel in a stratum above the gases from the burners i 8 and their temperature may be above the softening temperature of the ash when arriving at the boiler tubes. To avoid this possibility and also keep the auxiliary burners down to a practical number, applicant may, in some cases, employ a superheater of a size which, when contacted by all. the gases fromburners l8, would over-superheat at high boiler loads and provide a by-pass controlled by a damper 23 to divert some of the gas around the superheater in order tomaintain a desired constant superheat temperature. at these high loads. The upper set of burners 20 would then be employed as described aboveonly below that rate of operation where the superheat temperature falls below that desired. By way of example, 2200 degrees F. may be the softening temperature of the ash. below which the temperature of the flue gases should be kept upon entering the boiler tube bank. At peak loads the temperature of the gases leaving the furnace produced by burners i8 only may be 2100 degrees F. and the excess superheat is lowered by bypassing some of the gases. At normal load and proper superheat this temperature may be 2000 degrees F. At 60% of normal load this temperature may be 1700 degrees F. and, inasmuch as the temperature of the gases at the outlet from the furnace is in'sufiicient to give the desired superheat temperature the upper set of burners 20 are placed in operation and the firing of the burners it? reduced as described above. The fresh hot gases from the burners 20 would then raise the temperature of the mixture of gases from both sets of burners to about 1900 degrees F. to maintain the superheat temperature at the point desired. I

With this manner of operation atvery high loads the maintenance of a substantially constant suiperheated steam temperature over a range from around 60% of normal load to low loads is still attained by apportioning the fuel range of boiler loads since the fuel feeding means permit proper quantities vof gases to be produced ture; and, as the steam demand falls, decreasing.

at various loads and the total heat of the gases atthe outlet of the furnace maybe maintained at the temperatures required because the gases produced by the upper set of burners 20, not being cooled substantially before passing over the main generating and superheating element's, supply any deficiency of heat at low loads.

Although only one form of boiler for carrying -out the present invention has been illustrated herein it will be understood that the method may be carried out with various types of boilers having firing means arranged so that part of the gases produced are partially cooled in generating a portion of the required steam, whereas the remainder of the gases produced supply sufficient heat to attain the required gas temperature as a result of passing directly to the main generating and superheating elements without being substantially greater portion of the amount of fuel required to produce a quantity of gases at a temperature adequate to generate and superheat to a predetermined temperature the amount of steam required at a determined boiler load; imparting heat from the burning fuel to generating elements for generating part of the amount of steam required; separately burning any additional fuel required and mingling the hot gases therefrom with the gases that have been partially cooled by the extraction of heat therefrom; passing the mingled gases over other generating and the superheating elements to generate the remainder of the total amount of steam required and to superheat the latter to said predetermined temperathe amount of fuel burned to provide gases from which heat is extracted before passing over said other generating and said superheating elements while substantially proportionately increasing the amount of fuel burned to produce gases passed directly over said other generating and superheating elements for maintaining the gases pass- I ing over the latter heated to a temperature adequate to maintain said predetermined superheated steam temperature at low boiler loads.

- 2. The method of maintaining substantially constant superheated steam temperature over a wide range of loads in a boiler provided with a convection type superheater which comprises; burning at least the greater portion of the amount of fuel required to produce a quantity of gases at .said amount of steam; separately burning any additional fuel requiredand mingling the gases therefrom with the gases that have been partially cooled by the extraction of radiant heat therefrom; passing the mingled gases over other gen erating and the superheating elements to generate the remainder of the total amount of steam required and to superheat the latter to said predetermined temperature; and, as the steam demand falls, decreasing the amount of fuel burned to provide gases imparting radiant heat to first mentioned generating elements while increasing the amount of fuel burned to produce gases passed directly over said other generating and superheating elements for maintaining the gases passing over the latter heated to a temperature adequate to' maintain said predetermined superheated steam temperature at low boiler loads.

3. The method of maintaining substantially constant superheated .steam temperature over a Wide range of loads in a boiler provided with a convection type superheater which comprises; burning the total amount of fuel required to produce a quantity of gases at a temperature adequate to generate and superheat to a predetermined temperature the amount of steam required at maximum boiler loads; imparting heat from burning fuel to generating elements for generating part of the amount of steam required; passing the partially cooled gases over other generating and the superheating elements to generate the remainder of the total amount of steam required and to superheat the latter to said predetermined temperature; separately burning fuel at low loads and passing the gases therefrom directly over said other generating and the superheating elements; and; as the steam demand falls, decreasing the amount of fuel burned to provide gases heating said first mentioned generating elements while increasing the ainount of fuel burned toproduce gases passed directly over said other generating and superheating elements for maintaining the gases passing over the latter heated to a temperature adequate to maintain said predetermined superheated steam temperatureat low boiler loads.

4. The method of maintaining substantially constant superheated steam temperature over a wide range of steam demand in a boiler provided with a convection type superheater which comprises; burning a portion of the total amount of fuel required to produce a quantity of gases at a temperature adequate to generate and superheat to a predetermined temperature the amount of steam required at low loads, passing saidgases directly over the main generating and superheating elements of the boiler; separately burning the remainder of said amount of fuel and imparting heat therefrom to other steam generating elements before passing said gases over the main generating and superheating elements; and, upon increases in load, decreasing the portion of fuel burned to produce gases passed directly over said main generating and superheating elements while increasing the portion of fuel burned to produce gases from which heat is extracted to generate steam prior to being passed over said main generating and superheating elements.

5. The method of maintaining substantially constant superheated steam temperature in a boiler having separate sets of fuel burners near the top and bottom respectively of its furnace chamber, generating elements which absorb heat of burners with at least the greater portion of the total fuel required to produce gases at a temperature adequate to generate and superheat to a predetermined temperature the total amount of steam required at a determined boiler load; imparting heat from said gases to said generating elements which are heated only by the lower set of burners; passing the partially cooled gases from the furnace over said other generating and the superheating elements; supplying the upper set of burners with any additional fuel required to generate said amount of steam and attain said predetermined superheat temperature and passing the gases therefrom directly over said other generating and superheating elements; and as the steam demand falls, decreasing the amount of fuel supplied to the lower set of burners while increasing the amount of fuel supplied to the upper burners so that the temperature of gases is sufficiently high to maintain said predetermined superheated steam temperature at low loads.

6. The method of maintaining substantially constant superheated steam temperature in a boiler having separate sets of fuel burners near the top and bottom respectively of its furnace chamber, generating elements on the furnace walls which absorb heat only from the gases produced by the lower set of burners, and other generating and superheating elements in the path of gases from both sets of burners, which com- .prises; supplying the lower set of burners with at least the greater portion of the total fuel required to produce gases at a temperature adequate to generateand superheat to a predetermined temperature the total amount of steam required at a determined boiler load; imparting heat from said gases to said generating elements on the furnace walls; passing the partially cooled gases from the furnace over said other generating and the superheating elements; supplying the upper set of burners with any remaining fuel required to;

generate said amount of steam and attain said predetermined superheat temperature and passing the gases therefrom directly over said other generating and superheating elements; and, as the steam demand falls, decreasing the amount of fuel supplied to the lower set of burners while increasing the amount supplied to the upper burners so that the temperature of the gases is sufllciently high to maintain said predetermined superheated steam temperatures at low loads.

7. The method of maintaining a substantially.

constant superheated steam temperature in a boiler having a set of burners at or near the top of its furnace for generating a volume of gas which is passed directly over the main generating and the superheating elements of the boiler and another set of burners adjacent the bottom of the furnace arranged to impart radiant heat to metallic bodies to generate part of the steam before the gases therefrom mingle with those from the upper set of'burners to pass over said main generating and the superheating elements, which comprises; supplying the lower set of burners with the total fuel required to produce gases at a temperature to generate and superheat to a predetermined temperature the total amount of steam required at a determined boiler load; im-

determined temperature; supplying fuel to the upper set of burners at low loads and passing the gases therefrom directly over the main generating and the superheating elements; and, as the steam demand falls, decreasing the amount of fuel sup-' plied to the lower burners while increasing the amount of fuel supplied to the upper burners to maintain said gases heated to a temperature sufficiently high to attain said predetermined superheated steam temperature at low loads.

8. The method of maintaining substantially constant superheated steam temperature in a boiler having a furnace substantially uniformly surrounded by heat absorbing elements and a convection type superheater and other generating elements located beyond the furnace outlet and.

heated by gases therefrom which comprises; burning fuel in suspension in the lower portion of the furnace and imparting heat from the gases produced thereby to said heat absorbing elements; passing the gases from the furnace over said superheater and other generating elements; and,

varying the amount of heat absorbed by said heat absorbing elements by introducing fuel into the upper portion of the furnace and burning it therein while reducing the amount of fuel burned in the lower portion of the furnace to vary the ments located beyond the furnace outlet in the.

path of gases therefrom which comprises; introducing the total amount of fuel required to produce a quantity of gases adequate to generate and superheat to a predetermined temperature the amount of steam required at a determined boiler load through opposite walls in the lowerportion of the furnace in direction tangential to an imaginary circle within thefurnace for producing a vertical column of burning fuel and gases sweeping the heat absorbing elements on the furnace walls and yielding heat thereto by convection as well as by radiation to generate part of the amount of steam required; passing the partially cooled gases over the other generating and superheating elements to generate the remainder of the total amount of steam required and superheat the latter to said predetermined temperature; and, as the steam demand falls, separately burning fuel in the upper portion of the furnace and passing the gases therefrom directly over said other generating and the superheating elements and decreasing the amount of fuel introduced into the lower portion of the furnace for reducing the amount of heat imparted to said heat absorbing elements by con vection while maintaining the gases passing over said other generating and superheating elements heated to a temperature adequate to maintain said predetermined superheated steam temperature.

' 10. The method of maintaining asubstantially constant superheated steam temperature in a boiler having a furnace chamber provided with heat absorbing elements. and other generating elements and a convection type superheater located beyond the furnace outlet in the path of gases therefrom, which comprises; burning in said furnace an amount of fuel adequate to produce gases suficient in quantity and temperature to generate all of the steam required at maximum load but which would superheat the steam above'a desired, predetermined temperature; im-

-- erate the remainder of the steam while by-passing a suflicient part of said gases aroundthe superheater to maintain the superheated steam at said predetermined temperature; reducing the amount of fuel burned to produce gases impart ing heat to said heat absorbing elements as the steam demand falls; and, as the steam demand falls to a point below which the volume and temperature of the gases imparting heat to said heat absorbing elements are inadequate to maintain said predetermined, steam temperature, separately burning fuel to produce a volume of gases which do not impart asubstantial amount of heat to said heat absorbing elements; mixing said separately produced gases with the gases which have been partially cooled by imparting-heat to said heat absorbing elements for raising the temperature of the gases to'a point suiflciently high to superheat the steam to said predetermined temperature; and passing the entire volume of co-mingled gases over the superheater aswell as said other generating elements,

11. The method of maintaining a substantially constant superheated steam temperature in a boiler as recited in claim 10 wherein the amount of fuel producing gases imparting heat to "said heat absorbing elements is progressively decreased as the steam demand falls and the amount of fuel burned to produce gases not imparting heat to said heat absorbing elements is progressively increased as steam demand falls for maintaining the temperature of the comingled gases suillciently high to maintain said predetermined superheated steam temperature at low loads.

GEORGE P. JACKSON 

